In the handling of liquid slurries of compressible particulate material, particularly wood chips, sawdust, or like comminuted cellulosic fibrous material, oftentimes the material slurry is acted upon using a screen. Liquid is withdrawn from the slurry through the screen in a process, in the pulp art, often referred to as “extraction”. The withdrawn liquid may be passed to recovery (as in certain digester extractions), or may be reintroduced into the slurry, typically after treatment (e. g. increasing the temperature, removing undesirable materials, adding treatment or dilution liquids, or the like).
When liquid is withdrawn through a screen in cellulose pulp production or treatment, two significant phenomena occur, which are dealt with and utilized according to the invention. The first is compression, and the second “springback”.
Compaction toward the screen inherently occurs when liquid is drawn through a bed a compressible material (like comminuted cellulosic fibrous material) using a screen. The impact of the compaction means that the bulk of the extracted liquid will be at the first part of the screen surface in the direction of liquid flow (e. g. the top part of the screen when the slurry is flowing downwardly). The bulk of the friction force exerted on the screen by the flowing material will be at the second part of the screen surface in the direction of liquid flow since little liquid is being withdrawn there, and since the material is compacted at that location.
“Springback” refers to the phenomena that the material in the slurry tends to return to substantially its original permeability and consistency (density) if free to do so. That is, once the compaction forces are relieved, the comminuted cellulosic material will spring back to substantially its original permeability and consistency
The above principles are taken into account in the design of an improved diffuser, such as shown in co-pending application Ser. No. 09/306,416 filed May 6, 1999 (in particular FIGS. 13 and 14 thereof), the disclosure of which is hereby incorporated by reference herein. It has now been found, according to the present invention, that these same principles may be used in association with dynamic reactors in the pulp and paper art, such as continuous digesters (e. g. KAMYR digesters), such as shown—for example—in U.S. Pat. Nos. 5,620,562, 5,662,775, and 5,849,151, the disclosures of which are hereby incorporated by reference herein. That is, according to the present invention, there is provided a method and apparatus for restriction the time of extraction as the solids bed (slurry) moves past a screen, and to provide expansion after the screen, particularly when the slurry is flowing downwardly as in a continuous digester. This allows the use of gravity to provide a necessary force to overcome resistance on the screen, and to insure that the discharge from the device does not adversely impact the solids compaction at the screen.
According to a first aspect of the present invention, there is provided a method of treating comminuted cellulosic fibrous material in a liquid slurry, using a (preferably substantially stationary) screen having a screen surface with a plurality of parts, comprising:                (a) causing the slurry to move downwardly past the (e. g. substantially stationary) screen;        (b) withdrawing liquid from the slurry through a first part of the screen surface for a time period sufficient to avoid severe compaction;        (c) relieving the compaction force on the material of the slurry at a second part of the screen surface substantially immediately below the first part so as to encourage springback of the material; and        (d) repeating (b) and (c) at least once in following part of the screen surface as the slurry continues to move downwardly.        
In the method, wherein (b) and (c) may practiced so that the permeability of the material of the slurry does not increase to greater than about 750 lbs. per ft. squared per foot, and the time for (b) is dependent upon the actual material being treated, and differs, for example, for sawdust, conventional wood chips, etc. In one embodiment of the invention a)–(d) are practiced during continuous digesting of the comminuted cellulosic fibrous material, and the material may be a wide variety of materials, including sawdust, wood chips, bagesse, etc.
Also, (a)–(d) may be practiced at a first vertical position in a continuous treatment vessel, and then are repeated at a second vertical position in the continuous treatment vessel, vertically spaced from the first position. Further, (a)–(d) may be practiced using a screen having the screen surface thereof radially inwardly, or alternatively or in addition outwardly, of the moving slurry, and typically the method further comprises reintroducing the liquid withdrawn in (b) into the moving slurry substantially adjacent to where it was withdrawn, using conventional equipment (which may reheat the withdrawn liquid, and/or add digesting liquid or dilution liquid to it, or otherwise treat it as is conventional).
According to another aspect of the present invention, there is provided a method of continuously digesting comminuted cellulosic fibrous material in a slurry, using a (preferably substantially stationary) screen having a screen surface with a plurality of parts, comprising:                (a) substantially continuously treating the material with a digesting liquid;        (b) substantially continuously causing the slurry to move downwardly past the substantially stationary screen;        (c) substantially continuously withdrawing liquid from the slurry through a first part of the screen surface for a time period sufficient to avoid severe compaction; and        (d) substantially continuously relieving the compaction force on the material of the slurry at a second part of the screen surface substantially immediately below the first part so as to encourage springback of the material.        
In this aspect of the invention, (c) and (d) may be practiced so that the permeability of the material of the slurry does not increase to greater than about 750 lbs. per ft. squared per foot, and (a)–(d) may be practiced during continuous digesting of the comminuted cellulosic fibrous material.
According to yet another aspect of the present invention, there is provided a substantially upright vessel for treating or producing cellulose pulp, comprising: a vessel wall defining a substantially hollow interior, and having a top and bottom; at least one (preferably substantially stationary) screen having a screen surface with at least first and second parts, the first part above the second part and substantially immediately adjacent thereto; the first part being perforated, allowing the passage of extracted liquid therethrough, and defining a first cross-sectional area pathway for slurry flowing therepast; an extraction conduit operatively connected to the perforated first part; the second part being substantially solid and having a step-out substantially immediately adjacent the first part, so that the second part step-out defines a second cross-sectional area pathway greater than the first pathway; a slurry inlet adjacent the vessel wall top; and a slurry outlet adjacent the vessel wall bottom.
This aspect of the invention preferably further comprising a third perforated part of said screen surface substantially immediately adjacent and below said second part, and a fourth substantially solid part of said screen surface substantially immediately adjacent and below said third part and having a step-out; and optionally a perforated fifth part of said screen surface substantially immediately adjacent and below said fourth part. Typically both said first and third screen parts are operatively connected to said extraction conduit, and the extraction conduit may be passed to a recovery system (e. g. a black liquor handling system), and/or may recirculate withdrawn liquid into the vessel, e. g. after heating thereof, and removing, diluting, or adding to the liquid, as is conventional per se. The screen surface may be at or adjacent said vessel wall, and define a flow path of slurry radially inwardly thereof, and/or interior of the vessel wall and define a flow path radially outwardly thereof. A plurality of vertically space substantially stationary screen surfaces pay be provided in the vessel.
It is the primary object of the present invention to enhance the treatment of comminuted cellulosic fibrous material during continuous digesting, or treatment thereof where the material flows downwardly in a slurry. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.